Optical fiber gyros are now conventional practice.
In general, as shown diagrammatically in FIG. 1, they comprise a laser emitter module 1 and a detector module 2 connected via a coupler 7 and a Y junction 8 to a looped optical fiber 3, together with modulator means 4 disposed at the input/output ends of the loop, in the vicinity of the point--referenced A in FIG.1--where the ends of the loop 3 come together, said means 4 being controlled by a processor unit 5 to which the output signals from the module 2 are sent.
Such gyros make use of the Sagnac effect which occurs in the loop when the loop is subjected to rotation about its axis.
The wave rotating in the same direction of rotation as that imposed on the loop is subjected to a phase shift .phi..sub.1 of: EQU .phi..sub.1 =KL-2.pi.RL.OMEGA./.lambda.C
where:
K=wave number=2.pi./.lambda.
R=radius of the coil
L=optical path length in the fiber
.OMEGA.=angular velocity
.lambda.=wavelength
C=speed of light
while the wave rotating in the opposite direction is subjected to a phase shift .phi..sub.2 of: EQU .phi..sub.2 =KL+2.pi.RL.OMEGA./.lambda.C
The detector module 2 and the processor unit 5 make it possible, in theory, to measure the phase difference: EQU .phi..sub.2 -.phi..sub.1= 4.pi.RL.OMEGA./.lambda.C
from the output signal, which output signal corresponds to interference between the two waves when they return to point A after travelling along the full length of the coil.
Unfortunately, in reality, the term KL is much greater than 2.pi.RL.OMEGA./.lambda.C, and the ratio between them can be as great as 10.sup.15.
It will be understood that as a result the proper operation of optical fiber gyros requires the optical paths to present perfect reciprocity in both directions, i.e. the term KL in the equation for .phi..sub.1 must be exactly the same as the term KL in the equation for .phi..sub.2.
As a result, optical gyros normally operate only with monomode fibers that conserve polarization. With multimode fibers, the results obtained with a structure of the kind shown in FIG. 1 are disturbed by mixing and interference between the various modes, with each mode corresponding to a different value of L.
Unfortunately, polarization-conserving monomode fibers are much more expensive to manufacture than are multimode fibers.
The invention thus seeks to propose a multimode fiber gyro in which the various modes are recombined correctly, without their various waves interfering with one another.
Proposals have already been made to solve this problem of making multimode fiber gyros with phase conjugation.
It is recalled that phase-conjugating materials (wrongly called phase-conjugate "mirrors") are non-linear materials, e.g. photorefractive crystals, in which interference between an incident wave and a pumping wave creates refractive index gratings. The conjugate wave obtained at the outlet from such a crystal is the result of the pumping wave diffracting on said index gratings.
Unfortunately, the time required to build index gratings is very long compared with the time taken by light to pass through said gratings in photorefractive materials, such that any rapid variation in the parameters of the incident wave is absent from the conjugate wave, even though rapid variations in the parameters of the pumping wave are indeed to be found in the conjugate wave.
That is why the phase-conjugate gyros that have been proposed in the past have long response times.
For example, it can be shown that with phase-conjugate gyro structures of the kinds proposed in:
1! FR 2 503 862--"Dispositif optique interferometrique avec miroir(s) gyro conjugaison de phase, en particular gyrometre laser" Interferometer optical device with phase-conjugate mirror(s), in particular a laser gyro!, C. Borde, CNRS; and
2! Applied Optics, Vol. 25, No. 7, Apr. 1, 1986, "Phase conjugate optic gyro", a phase shift of .DELTA..phi. in the gyro loop that is established for a length of time that is shorter than the response time of the phase-conjugating crystal gives rise, in said phase shift, whether reciprocal or otherwise, to a phase difference of .DELTA..phi. or of--.DELTA..phi. on the detected outlet signal, whereas for a phase shift established for a duration greater than the response time of the crystal, the phase difference in the detected output signal is 2.DELTA..phi..
Similarly, with self-pumped phase-conjugate mirror gyro structures, such as those proposed in:
3! EP 79268 --Michelson type interferometer with a photorefractive mirror.
4! Optics Letters, Vol. 11, No. 10, Oct. 1986, "Self-pumped phase-conjugate fiber optic gyro",
5! Optics Letters, Vol. 12, No. 12, Dec. 1987, "Phase-conjugate multimode fiber gyro", C*W. H. Chen, P. J. Wang;
6! SPIE, Vol. 838, Fiber optic and laser sensors, 1987, "Phase-conjugate fiber optic gyro with multimode fibers"; and
7! Yasuo Tamita, IEEE Journal of quantum electronics, Vol. 25, No. 3, Mar. 1989, "Polarization and spatial recovery by modal dispersal and phase conjugation: properties and applications", a .DELTA..phi. phase shift in the gyro loop established during a length of time that is shorter than the response time of the phase-conjugating crystal gives rise, depending on whether said phase shift is reciprocal or not, to a phase difference of 2.DELTA..phi. or of 0 in the output signal as detected, whereas for a phase shift that is established for a length of time greater than the response time of the crystal, the phase difference in the detected signal is 2.DELTA..phi..
Consequently, prior art gyros and phase-conjugate gyros are incapable of operating with signals at a frequency greater than the frequency which corresponds to the response time of the phase-conjugating crystal.
In particular, such large response times prevent such phase-conjugate gyros being used with phase shift type modulation such as that implemented by the processor unit 5 and the modulator means 4 of the structure of FIG. 1, since such phase shift keying requires fast response times.
Unfortunately, processing by means of phase shift, which makes it possible to ignore the high degree of power non-linearity in the detectors as a function of phase, turns out to be essential for proper operation of present gyros.